3.1.4 DrawingML Persistence Part
When Word creates a legacy shape from an OfficeArt object, additional information will be stored in the legacy shape to allow Word to reconstruct the original OfficeArt object at a later time. This information is stored in the gfxdata attribute ("[ECMA-376] Part 4 §188.8.131.52; shape").
The contents of the gfxdata attribute is an xsd:base64Binary representation of a Package ("[ECMA-376] Part 2, Open Packaging Conventions"). This package contains information about the original OfficeArt object as well as information about the characteristics of the legacy shape as it was originally created to represent the OfficeArt object.